The present invention relates to electric motor driven air compressors of sliding vane eccentric rotor type and is concerned with minimising the power consumption of such compressors when the compressor is subjected to a varying demand for compressed air.
Compressors of this type are well known and are disclosed in numerous prior documents such as British Patent No. 1318884. Such compressors are normally driven by four pole asynchronous electric motors at a speed of a little less than 1500 rpm with a 50 Hz power supply and 1800 rpm with a 60 Hz power supply. It is desirable for reasons for minimum use of energy to ensure that the output of the compressor matches the demand for compressed air as closely as possible and numerous ways of doing this are known. The simplest and best known way of doing this is to provide the compressor with a simple unloader valve and such a valve is disclosed in British Patent No. 1318884. An unloader valve comprises a valve cooperating with the compressor inlet and operated by a servo device which is subjected to the compressor delivery pressure. As the delivery pressure rises above the normal value, thereby indicating that the demand for compressed air is less than the rate at which it is being supplied, the servo device causes the unloader valve to move progressively to throttle the compressor inlet and thus to reduce the supply of compressed air. However, as the inlet is throttled, the pressure at the inlet of the compressor falls to a sub-atmospheric value and this means that the pressure differential across the vanes of the compressor increases and the discharge pressure of the compressor increases. This results in more energy being required to rotate the rotor and it is found in practice that in a conventional compressor provided with an unloader valve the energy consumption when the demand for compressed air is zero is approximately 70% of the energy consumption when the compressor is producing its nominal rated output.
A more sophisticated system for minimising power consumption is disclosed in British Patent No. 1599319 in which the compressor is provided not only with an unloader valve of the type referred to above but also with a minimum pressure valve which is arranged selectively to close the outlet and a vent valve arranged to vent the interior of the compressor, all of which are connected to a controller. The controller includes a timer and when the unloader valve has been closed for a predetermined period of time, thereby indicating that the demand for compressed air has been zero for that period of time, the minimum pressure valve is closed whilst maintaining the unloader valve closed and the vent valve is opened. The interior of the compressor is then vented down to a predetermined low pressure which results in a significant decrease in the power consumed when there is no demand for compressed air. However, it is not possible to reduce the pressure in the compressor to zero, since a certain minimum pressure is necessary in order to inject oil from the compressor sump into the interior of the stator. Furthermore, if the demand for compressor air is in fact not zero but is at a relatively low level the compressor will cycle repeatedly between the full load condition and the vented down condition and the repeated venting down of the interior of the compressor followed by the necessity of repressurising the interior of the compressor results in the compressor in accordance with British Patent No. 1599319 still consuming a substantial proportion of the full load power consumption even when the demand for compressed air is at only a small fraction of the full rated load.
Economy of operation is becoming increasingly important and it is therefore the object of the invention to provide an air compressor of sliding vane eccentric rotor type in which the energy consumption is substantially reduced, when the compressor is subjected to a varying demand for compressed air, by comparison with that disclosed in British Patent No. 1599319.
One superficially attractive way of varying the output of the compressor to match changes in demand and simultaneously reduce the power consumed would be to change the speed of the motor and this is particularly attractive for a quite separate reason, namely that in contrast to compressors of screw type, whose efficiency may drop when their speed drops, the efficiency of a compressor of sliding vane eccentric rotor type rises somewhat as the speed drops. This increase in efficiency with reduced speed is however only achieved down to a critical minimum speed of something less than 1000 rpm because below this value instability or chatter of the blades sets in. Thus at speeds significantly below 1000 rpm the centrifugal force acting on the blades and tending to force them into contact with the internal surface of the bore in the stator is insufficient to withstand the pressure differential across the blades which therefore repeatedly lift away briefly from the surface of the stator bore and permit leakage of compressed air between the two compression cells which they separate. It is also found empirically that it is not efficient to operate compressors of sliding vane eccentric rotor type at speeds much greater than 1500 rpm because if they are operated at such a speed the increase in the contact pressure between the vanes and the surface of the stator bore caused by the increase in the centrifugal force acting on the vanes results in increasing frictional losses and thus in decreasing mechanical efficiency. There is, therefore, in practice a relatively narrow speed range in which such compressors must be operated, that is to say between something above 1500 rpm and something below 1000 rpm.
Whilst it would in theory be possible to use a variable speed motor and to vary the speed of the motor within the range set forth above in order to match the output of the compressor to the demand for compressed air, it is found that the capital cost of such motors and their attendant control systems is unacceptably high. A further possibility would be to provide variable gearing between the compressor and a constant speed motor but such gearing is also unacceptably expensive. For these various reasons, no significant progress has therefore been made with variable speed compressors of sliding vane eccentric rotor type.
However, pole amplitude modulated (PAM) motors have recently become available. Such motors are manufactured and sold by Brook Hansen and others. Such motors may be switched from four pole to six pole operation, thereby changing their speed from a little under 1500 rpm to a little under 1000 rpm, by altering the position of the electrical supply connections. The use of such PAM motors in connection with sliding vane compressors in order to produce a variable speed compressor is therefore superficially very attractive but they are in practice not as attractive as would be expected, due in part to the substantial noise which is generated when the motor is switched between high and low speeds and, more importantly, due to the fact that there is a substantial, though brief, current surge when switching below high and low speeds. This current surge leads to a brief substantial drop in voltage of the power supply and this is not only disconcerting in that it tends to result in flickering of the lighting but is also highly disruptive in that it can lead to malfunctioning of sensitive electronic equipment, such as computers, and is also potentially dangerous in that, for instance, machine tools incorporating magnetic chucks may lose control of their work piece. These problems currently render the use of PAM motors in conjunction with compressors unacceptable and the only way in which these problems could be overcome would be by installing a much higher rated power supply and this would be wholly unacceptable due to the very substantial cost involved.